Actually it's the front derailleur that requires the substantially more powerful servo. It takes quite a bit of grunt to mash the chain across such a large size difference between the two rings. Shimano's battery life (single charge) figures estimate 35,000 rear shifts, but only 7,500 front shifts, due to the greater power needs at the front. When the battery gets low, front shifting stops first, to reduce the drain on the system, allowing you a better chance to get home with at least some range of gears.

As others have mentioned wireless stuff malfunctions easily due to electrical interference. I used to have a training route that went near an airport and at one particular place my computer would always stop, probably due to the radar or some such. I also had a heart rate monitor that would always show my heart rate dropping to about half when I was riding alongside a particular girl and I'm pretty sure that wasn't happening in reality.

As soon as wireless gears come out, someone will be working out a way of interfering with or overriding the signal.

Instead of protesters throwing tacks on the road like they did at the Tour a couple of years ago they'll set up a transmitter at the base of a climb and prevent anyone from changing gears, or even changing gears for them. Electronic nobbling!